In recent years, in an optical network field applying WDM technology, it becomes possible to establish an optical network having a complex topology, such as a ring interconnection network or a mesh network, by using an apparatus such as an optical add-drop multiplexing (OADM) device which realizes the adding/dropping of optical signals without any opto-electronic conversion at a wavelength unit and the changing-over the optical signal path.
In an optical network having such a complex topology, paths through which optical signals of different wavelength of WDM light are passed (i.e., wavelength paths) have diversified, so that more stringent demand for quality is imposed on each optical signal which reaches a terminal node (receiving end). To satisfy this demand, it is important to design a level diagram of a WDM light within an optical network according to intended network architecture. Furthermore, it was also effective to realize a desired received signal quality by enhancing an error correction performance when subjecting an optical signal received at a terminal node to an error correction process for correcting a code error.
A conventional level diagram of WDM light within an optical network is typically designed so that an optical signal power at each wavelength is to be the same for each wavelength and for each span between nodes. Furthermore, as disclosed in Japanese Laid-Open Patent Publication No. 2001-203414 and Japanese Laid-Open Patent Publication No. 11-8590, a pre-emphasis scheme is also known for controlling signal levels of WDM light at a transmitting end for each wavelength so that characteristics such as optical signal-to-noise ratios (OSNRs) at a receiving end are to be equal for each wavelength.
Setting a transmission power corresponding to each wavelength of WDM light by the aforementioned pre-emphasis scheme allows the qualities of the received optical signals of all wavelengths to be maintained at an equivalent level. Furthermore, individually applying an error correction process having a desired performance to the received signal of each wavelength ensures receipt of optical signals of various wavelengths which are passed through various wavelength paths.
However, when the received signal qualities of optical signals of each wavelength received at a terminal node are maintained at an equivalent level without depending on wavelength in the above-described optical network of prior art, it is necessary to uniformly apply an error correction process for all wavelengths. As a result, increase in power consumption of the overall optical network poses a problem.
Specifically, enhancement of error correction performance requires an error correction circuit module such as LSI (Large Scale Integration) which performs an error correction process to conduct a more complex operation. Therefore, the circuit module for the error correction process tends to increase power consumption with increase in gate size. When the error correction process is uniformly performed on all wavelengths of the WDM light, it is required to provide and drive as many circuit modules having a desired error correction ability as the wavelengths of the WDM light. Therefore, the power consumption of the overall optical network increases with increase in number of wavelengths of the WDM light and with enhancement of error correction performance.
In the optical network having a complex topology as described above, a traffic volume in the optical network may significantly vary when optical signals are added or dropped at two or more OADM nodes. In this case, in a repeating section (span) which can afford a relatively sufficient traffic volume, the received signal quality of optical signals can be improved by setting higher than usual a level of an optical signal which can pass through the span. In other words, in a span in which the number of wavelengths of the WDM light to be transmitted between OADM nodes decreases, even if the optical signal power of each wavelength to be fed to a transmission line slightly increases, the total WDM light power does not increase as much as that which causes signal deterioration due to a nonlinear optical effect on the transmission line. Thus, a signal quality at a receiving end is improved by signal level increase at the span.
When focusing on this point, a received signal quality associated with a specific wavelength could be improved more than the other wavelengths by setting a level diagram in consideration of traffic volume variations within an optical network. If a received signal quality associated with a particular wavelength is improved, the particular wavelength merely needs an error correction process with relatively lower error correction ability or needs no error correction process, so that reduction in power consumption of the overall optical network can be expected. However, in the WDM light level diagram according to the conventional pre-emphasis scheme mentioned above, an optical signal power of each wavelength is set at a transmitting end and no level adjustment for each wavelength is performed at a node disposed partway along the transmission line, so that it is difficult to realize a level diagram which takes into account the traffic volume variations in an optical network as described above.